ipfw [-acdefnNStT] [set N] {list | show} [rule | firstlast ...]
     ipfw [-f | -q] [set N] flush
     ipfw [-q] [set N] {delete | zero | resetlog} [number ...]
     ipfw enable
	  {firewall | altq | one_pass | debug | verbose | dyn_keepalive}
     ipfw disable
	  {firewall | altq | one_pass | debug | verbose | dyn_keepalive}

     ipfw set [disable number ...] [enable number ...]
     ipfw set move [rule] number to number
     ipfw set swap number number
     ipfw set show

     ipfw table number add addr[/masklen] [value]
     ipfw table number delete addr[/masklen]
     ipfw table {number | all} flush
     ipfw table {number | all} list

     ipfw {pipe | queue} number config configoptions
     ipfw [-s [field]] {pipe | queue} {delete | list | show} [number ...]

     ipfw [-q] nat number config configoptions

     ipfw [-cfnNqS] [-p preproc [preprocflags]] pathname


     The ipfw utility is the user interface for controlling the ipfw(4) fire‐
     wall and the dummynet(4) traffic shaper in FreeBSD.

     An ipfw configuration, or ruleset, is made of a list of rules numbered
     from 1 to 65535.  Packets are passed to ipfw from a number of different
     places in the protocol stack (depending on the source and destination of
     the packet, it is possible that ipfw is invoked multiple times on the
     same packet).  The packet passed to the firewall is compared against each
     of the rules in the firewall ruleset.  When a match is found, the action
     corresponding to the matching rule is performed.

     Depending on the action and certain system settings, packets can be rein‐
     jected into the firewall at some rule after the matching one for further

     An ipfw ruleset always includes a default rule (numbered 65535) which
     cannot be modified or deleted, and matches all packets.  The action asso‐
     ciated with the default rule can be either deny or allow depending on how
     the kernel is configured.

     If the ruleset includes one or more rules with the keepstate or limit
     option, ipfw will have a stateful behaviour, i.e., upon a match it will
     create dynamic rules matching the exact parameters (source and destina‐
     tion addresses and ports) of the matching packet.

     These dynamic rules, which have a limited lifetime, are checked at the
     Section SETS OF RULES for more information on sets.

     Rules can be added with the add command; deleted individually or in
     groups with the delete command, and globally (except those in set 31)
     with the flush command; displayed, optionally with the content of the
     counters, using the show and list commands.  Finally, counters can be
     reset with the zero and resetlog commands.

     The following general options are available when invoking ipfw:

     -a      Show counter values when listing rules.  The show command implies
	     this option.

     -b      Only show the action and the comment, not the body of a rule.
	     Implies -c.

     -c      When entering or showing rules, print them in compact form, i.e.,
	     omitting the "ip from any to any" string when this does not carry
	     any additional information.

     -d      When listing, show dynamic rules in addition to static ones.

     -e      When listing and -d is specified, also show expired dynamic

     -f      Do not ask for confirmation for commands that can cause problems
	     if misused, i.e. flush.  If there is no tty associated with the
	     process, this is implied.

     -i      When listing a table (see the LOOKUP TABLES section below for
	     more information on lookup tables), format values as IP
	     addresses. By default, values are shown as integers.

     -n      Only check syntax of the command strings, without actually pass‐
	     ing them to the kernel.

     -N      Try to resolve addresses and service names in output.

     -q      Be quiet when executing the add, nat, zero, resetlog or flush
	     commands; (implies -f).  This is useful when updating rulesets by
	     executing multiple ipfw commands in a script (e.g.,
	     ‘sh /etc/rc.firewall’), or by processing a file with many ipfw
	     rules across a remote login session.  It also stops a table add
	     or delete from failing if the entry already exists or is not

	     The reason why this option may be important is that for some of
	     these actions, ipfw may print a message; if the action results in
	     blocking the traffic to the remote client, the remote login ses‐
	     sion will be closed and the rest of the ruleset will not be pro‐
	     cessed.  Access to the console would then be required to recover.

     To ease configuration, rules can be put into a file which is processed
     using ipfw as shown in the last synopsis line.  An absolute pathname must
     be used.  The file will be read line by line and applied as arguments to
     the ipfw utility.

     Optionally, a preprocessor can be specified using -p preproc where
     pathname is to be piped through.  Useful preprocessors include cpp(1) and
     m4(1).  If preproc does not start with a slash (‘/’) as its first charac‐
     ter, the usual PATH name search is performed.  Care should be taken with
     this in environments where not all file systems are mounted (yet) by the
     time ipfw is being run (e.g. when they are mounted over NFS).  Once -p
     has been specified, any additional arguments are passed on to the pre‐
     processor for interpretation.  This allows for flexible configuration
     files (like conditionalizing them on the local hostname) and the use of
     macros to centralize frequently required arguments like IP addresses.

     The ipfw pipe and queue commands are used to configure the traffic
     shaper, as shown in the TRAFFIC SHAPER (DUMMYNET) CONFIGURATION Section

     If the world and the kernel get out of sync the ipfw ABI may break, pre‐
     venting you from being able to add any rules.  This can adversely effect
     the booting process.  You can use ipfw disable firewall to temporarily
     disable the firewall to regain access to the network, allowing you to fix
     the problem.


     A packet is checked against the active ruleset in multiple places in the
     protocol stack, under control of several sysctl variables.  These places
     and variables are shown below, and it is important to have this picture
     in mind in order to design a correct ruleset.

		  ^    to upper layers	  V
		  |			  |
		  ^			  V
	    [ip(6)_input]	    [ip(6)_output]     net.inet(6).ip(6).fw.enable=1
		  |			  |
		  ^			  V
	    [ether_demux]	 [ether_output_frame]  net.link.ether.ipfw=1
		  |			  |
		  +‐‐>‐‐[bdg_forward]‐‐>‐‐+	       net.link.bridge.ipfw=1
		  ^			  V
		  |	 to devices	  |

     As can be noted from the above picture, the number of times the same
     packet goes through the firewall can vary between 0 and 4 depending on
     packet source and destination, and system configuration.

     Note that as packets flow through the stack, headers can be stripped or
     added to it, and so they may or may not be available for inspection.
     E.g., incoming packets will include the MAC header when ipfw is invoked

	   ipfw add 10 skipto 1000 all from any to any layer2 in
	   # packets from ip_input
	   ipfw add 10 skipto 2000 all from any to any not layer2 in
	   # packets from ip_output
	   ipfw add 10 skipto 3000 all from any to any not layer2 out
	   # packets from ether_output_frame
	   ipfw add 10 skipto 4000 all from any to any layer2 out

     (yes, at the moment there is no way to differentiate between ether_demux
     and bdg_forward).


     In general, each keyword or argument must be provided as a separate com‐
     mand line argument, with no leading or trailing spaces.  Keywords are
     case‐sensitive, whereas arguments may or may not be case‐sensitive
     depending on their nature (e.g. uid’s are, hostnames are not).

     In ipfw2 you can introduce spaces after commas ’,’ to make the line more
     readable.	You can also put the entire command (including flags) into a
     single argument.  E.g., the following forms are equivalent:

	   ipfw ‐q add deny src‐ip,
	   ipfw ‐q add deny src‐ip,
	   ipfw "‐q add deny src‐ip,"


     The format of ipfw rules is the following:

	   [rule_number] [set set_number] [prob match_probability] action
	   [log [logamount number]] [altq queue] [{tag | untag} number] body

     where the body of the rule specifies which information is used for fil‐
     tering packets, among the following:

	Layer‐2 header fields		      When available
	IPv4 and IPv6 Protocol		      TCP, UDP, ICMP, etc.
	Source and dest. addresses and ports
	Direction			      See Section PACKET FLOW
	Transmit and receive interface	      By name or address
	Misc. IP header fields		      Version, type of service, data‐
					      gram length, identification,
					      fragment flag (non‐zero IP off‐
					      set), Time To Live
	IP options
	IPv6 Extension headers		      Fragmentation, Hop‐by‐Hop
					      options, Routing Headers, Source
					      routing rthdr0, Mobile IPv6
					      rthdr2, IPSec options.
	IPv6 Flow‐ID
	Misc. TCP header fields 	      TCP flags (SYN, FIN, ACK, RST,
					      etc.), sequence number, acknowl‐
					      edgment number, window

     alone might not guarantee the desired results.

	     Each rule is associated with a rule_number in the range 1..65535,
	     with the latter reserved for the default rule.  Rules are checked
	     sequentially by rule number.  Multiple rules can have the same
	     number, in which case they are checked (and listed) according to
	     the order in which they have been added.  If a rule is entered
	     without specifying a number, the kernel will assign one in such a
	     way that the rule becomes the last one before the default rule.
	     Automatic rule numbers are assigned by incrementing the last non‐
	     default rule number by the value of the sysctl variable
	     net.inet.ip.fw.autoinc_step which defaults to 100.  If this is
	     not possible (e.g. because we would go beyond the maximum allowed
	     rule number), the number of the last non‐default value is used

     set set_number
	     Each rule is associated with a set_number in the range 0..31.
	     Sets can be individually disabled and enabled, so this parameter
	     is of fundamental importance for atomic ruleset manipulation.  It
	     can be also used to simplify deletion of groups of rules.	If a
	     rule is entered without specifying a set number, set 0 will be
	     Set 31 is special in that it cannot be disabled, and rules in set
	     31 are not deleted by the ipfw flush command (but you can delete
	     them with the ipfw delete set 31 command).  Set 31 is also used
	     for the default rule.

     prob match_probability
	     A match is only declared with the specified probability (floating
	     point number between 0 and 1).  This can be useful for a number
	     of applications such as random packet drop or (in conjunction
	     with dummynet) to simulate the effect of multiple paths leading
	     to out‐of‐order packet delivery.

	     Note: this condition is checked before any other condition,
	     including ones such as keep‐state or check‐state which might have
	     side effects.

     log [logamount number]
	     When a packet matches a rule with the log keyword, a message will
	     be logged to syslogd(8) with a LOG_SECURITY facility.  The log‐
	     ging only occurs if the sysctl variable net.inet.ip.fw.verbose is
	     set to 1 (which is the default when the kernel is compiled with
	     IPFIREWALL_VERBOSE) and the number of packets logged so far for
	     that particular rule does not exceed the logamount parameter.  If
	     no logamount is specified, the limit is taken from the sysctl
	     variable net.inet.ip.fw.verbose_limit.  In both cases, a value of
	     0 removes the logging limit.

	     Once the limit is reached, logging can be re‐enabled by clearing
	     packet can have multiple tags at the same time.  Tags are
	     "sticky", meaning once a tag is applied to a packet by a matching
	     rule it exists until explicit removal.  Tags are kept with the
	     packet everywhere within the kernel, but are lost when packet
	     leaves the kernel, for example, on transmitting packet out to the
	     network or sending packet to a divert(4) socket.

	     To check for previously applied tags, use the tagged rule option.
	     To delete previously applied tag, use the untag keyword.

	     Note: since tags are kept with the packet everywhere in ker‐
	     nelspace, they can be set and unset anywhere in the kernel net‐
	     work subsystem (using the mbuf_tags(9) facility), not only by
	     means of the ipfw(4) tag and untag keywords.  For example, there
	     can be a specialized netgraph(4) node doing traffic analyzing and
	     tagging for later inspecting in firewall.

     untag number
	     When a packet matches a rule with the untag keyword, the tag with
	     the number number is searched among the tags attached to this
	     packet and, if found, removed from it.  Other tags bound to
	     packet, if present, are left untouched.

     altq queue
	     When a packet matches a rule with the altq keyword, the ALTQ
	     identifier for the given queue (see altq(4)) will be attached.
	     Note that this ALTQ tag is only meaningful for packets going
	     "out" of IPFW, and not being rejected or going to divert sockets.
	     Note that if there is insufficient memory at the time the packet
	     is processed, it will not be tagged, so it is wise to make your
	     ALTQ "default" queue policy account for this.  If multiple altq
	     rules match a single packet, only the first one adds the ALTQ
	     classification tag.  In doing so, traffic may be shaped by using
	     count altq queue rules for classification early in the ruleset,
	     then later applying the filtering decision.  For example,
	     checkstate and keepstate rules may come later and provide the
	     actual filtering decisions in addition to the fallback ALTQ tag.

	     You must run pfctl(8) to set up the queues before IPFW will be
	     able to look them up by name, and if the ALTQ disciplines are
	     rearranged, the rules in containing the queue identifiers in the
	     kernel will likely have gone stale and need to be reloaded.
	     Stale queue identifiers will probably result in misclassifica‐

	     All system ALTQ processing can be turned on or off via ipfw
	     enable altq and ipfw disable altq.  The usage of
	     net.inet.ip.fw.one_pass is irrelevant to ALTQ traffic shaping, as
	     the actual rule action is followed always after adding an ALTQ

     count   Update counters for all packets that match rule.  The search con‐
	     tinues with the next rule.

     deny | drop
	     Discard packets that match this rule.  The search terminates.

     divert port
	     Divert packets that match this rule to the divert(4) socket bound
	     to port port.  The search terminates.

     fwd | forward ipaddr | tablearg[,port]
	     Change the next‐hop on matching packets to ipaddr, which can be
	     an IP address or a host name.  The next hop can also be supplied
	     by the last table looked up for the packet by using the tablearg
	     keyword instead of an explicit address.  The search terminates if
	     this rule matches.

	     If ipaddr is a local address, then matching packets will be for‐
	     warded to port (or the port number in the packet if one is not
	     specified in the rule) on the local machine.
	     If ipaddr is not a local address, then the port number (if speci‐
	     fied) is ignored, and the packet will be forwarded to the remote
	     address, using the route as found in the local routing table for
	     that IP.
	     A fwd rule will not match layer‐2 packets (those received on
	     ether_input, ether_output, or bridged).
	     The fwd action does not change the contents of the packet at all.
	     In particular, the destination address remains unmodified, so
	     packets forwarded to another system will usually be rejected by
	     that system unless there is a matching rule on that system to
	     capture them.  For packets forwarded locally, the local address
	     of the socket will be set to the original destination address of
	     the packet.  This makes the netstat(1) entry look rather weird
	     but is intended for use with transparent proxy servers.

	     To enable fwd a custom kernel needs to be compiled with the
	     option options IPFIREWALL_FORWARD.

     nat nat_nr
	     Pass packet to a nat instance (for network address translation,
	     address redirect, etc.): see the NETWORK ADDRESS TRANSLATION
	     (NAT) Section for further information.

     pipe pipe_nr
	     Pass packet to a dummynet “pipe” (for bandwidth limitation,
	     Section for further information.  The search terminates; however,
	     on exit from the pipe and if the sysctl(8) variable
	     net.inet.ip.fw.one_pass is not set, the packet is passed again to
	     the firewall code starting from the next rule.

     queue queue_nr
	     Skip all subsequent rules numbered less than number.  The search
	     continues with the first rule numbered number or higher.  It is
	     possible to use the tablearg keyword with a skipto for a computed
	     skipto, but care should be used, as no destination caching is
	     possible in this case so the rules are always walked to find it,
	     starting from the skipto.

     tee port
	     Send a copy of packets matching this rule to the divert(4) socket
	     bound to port port.  The search continues with the next rule.

     unreach code
	     Discard packets that match this rule, and try to send an ICMP
	     unreachable notice with code code, where code is a number from 0
	     to 255, or one of these aliases: net, host, protocol, port,
	     needfrag, srcfail, netunknown, hostunknown, isolated,
	     netprohib, hostprohib, tosnet, toshost, filterprohib,
	     hostprecedence or precedencecutoff.  The search terminates.

     unreach6 code
	     Discard packets that match this rule, and try to send an ICMPv6
	     unreachable notice with code code, where code is a number from 0,
	     1, 3 or 4, or one of these aliases: noroute, adminprohib,
	     address or port.  The search terminates.

     netgraph cookie
	     Divert packet into netgraph with given cookie.  The search termi‐
	     nates.  If packet is later returned from netgraph it is either
	     accepted or continues with the next rule, depending on
	     net.inet.ip.fw.one_pass sysctl variable.

     ngtee cookie
	     A copy of packet is diverted into netgraph, original packet is
	     either accepted or continues with the next rule, depending on
	     net.inet.ip.fw.one_pass sysctl variable.  See ng_ipfw(4) for more
	     information on netgraph and ngtee actions.

     setfib fibnum
	     The packet is tagged so as to use the FIB (routing table) fibnum
	     in any subsequent forwarding decisions.  Initially this is lim‐
	     ited to the values 0 through 15, see setfib(8).  Processing con‐
	     tinues at the next rule.

     reass   Queue and reassemble ip fragments.  If the packet is not frag‐
	     mented, counters are updated and processing continues with the
	     next rule.  If the packet is the last logical fragment, the
	     packet is reassembled and, if net.inet.ip.fw.one_pass is set to
	     0, processing continues with the next rule, else packet is
	     allowed to pass and search terminates.  If the packet is a frag‐
	     ment in the middle, it is consumed and processing stops immedi‐

	     is all you need at the beginning of your ruleset.

     The body of a rule contains zero or more patterns (such as specific
     source and destination addresses or ports, protocol options, incoming or
     outgoing interfaces, etc.)  that the packet must match in order to be
     recognised.  In general, the patterns are connected by (implicit) and
     operators ‐‐ i.e., all must match in order for the rule to match.	Indi‐
     vidual patterns can be prefixed by the not operator to reverse the result
     of the match, as in

	   ipfw add 100 allow ip from not to any

     Additionally, sets of alternative match patterns (orblocks) can be con‐
     structed by putting the patterns in lists enclosed between parentheses (
     ) or braces { }, and using the or operator as follows:

	   ipfw add 100 allow ip from { x or not y or z } to any

     Only one level of parentheses is allowed.	Beware that most shells have
     special meanings for parentheses or braces, so it is advisable to put a
     backslash \ in front of them to prevent such interpretations.

     The body of a rule must in general include a source and destination
     address specifier.  The keyword any can be used in various places to
     specify that the content of a required field is irrelevant.

     The rule body has the following format:

	   [proto from src to dst] [options]

     The first part (proto from src to dst) is for backward compatibility with
     earlier versions of FreeBSD.  In modern FreeBSD any match pattern
     (including MAC headers, IP protocols, addresses and ports) can be speci‐
     fied in the options section.

     Rule fields have the following meaning:

     proto: protocol | { protocol or ... }

     protocol: [not] protocolname | protocolnumber
	     An IP protocol specified by number or name (for a complete list
	     see /etc/protocols), or one of the following keywords:

	     ip4 | ipv4
		     Matches IPv4 packets.

	     ip6 | ipv6
		     Matches IPv6 packets.

	     ip | all

     addr: [not] {any | me | me6 | table(number[,value]) | addrlist |

     any     matches any IP address.

     me      matches any IP address configured on an interface in the system.

     me6     matches any IPv6 address configured on an interface in the sys‐
	     tem.  The address list is evaluated at the time the packet is an‐

	     Matches any IPv4 address for which an entry exists in the lookup
	     table number.  If an optional 32‐bit unsigned value is also spec‐
	     ified, an entry will match only if it has this value.  See the
	     LOOKUP TABLES section below for more information on lookup

     addrlist: ipaddr[,addrlist]

	     A host or subnet address specified in one of the following ways:

	     numericip | hostname
		     Matches a single IPv4 address, specified as dotted‐quad
		     or a hostname.  Hostnames are resolved at the time the
		     rule is added to the firewall list.

		     Matches all addresses with base addr (specified as an IP
		     address, a network number, or a hostname) and mask width
		     of masklen bits.  As an example, or
		     will match all IP numbers from to .

		     Matches all addresses with base addr (specified as an IP
		     address, a network number, or a hostname) and the mask of
		     mask, specified as a dotted quad.	As an example, or will match
		     1.*.3.*.  This form is advised only for non‐contiguous
		     masks.  It is better to resort to the addr/masklen format
		     for contiguous masks, which is more compact and less

     addrset: addr[/masklen]{list}

     list: {num | numnum}[,list]
	     Matches all addresses with base address addr (specified as an IP
	     address, a network number, or a hostname) and whose last byte is
	     in the list between braces { } .  Note that there must be no spa‐
	     ces between braces and numbers (spaces after commas are allowed).

	     A host or subnet specified one of the following ways:

	     numericip | hostname
		     Matches a single IPv6 address as allowed by inet_pton(3)
		     or a hostname.  Hostnames are resolved at the time the
		     rule is added to the firewall list.

		     Matches all IPv6 addresses with base addr (specified as
		     allowed by inet_pton or a hostname) and mask width of
		     masklen bits.

	     No support for sets of IPv6 addresses is provided because IPv6
	     addresses are typically random past the initial prefix.

     ports: {port | portport}[,ports]
	     For protocols which support port numbers (such as TCP and UDP),
	     optional ports may be specified as one or more ports or port
	     ranges, separated by commas but no spaces, and an optional not
	     operator.	The ‘‐’ notation specifies a range of ports (including

	     Service names (from /etc/services) may be used instead of numeric
	     port values.  The length of the port list is limited to 30 ports
	     or ranges, though one can specify larger ranges by using an
	     orblock in the options section of the rule.

	     A backslash (‘\’) can be used to escape the dash (‘‐’) character
	     in a service name (from a shell, the backslash must be typed
	     twice to avoid the shell itself interpreting it as an escape

		   ipfw add count tcp from any ftp\\‐data‐ftp to any

	     Fragmented packets which have a non‐zero offset (i.e., not the
	     first fragment) will never match a rule which has one or more
	     port specifications.  See the frag option for details on matching
	     fragmented packets.

     Additional match patterns can be used within rules.  Zero or more of
     these so‐called options can be present in a rule, optionally prefixed by
     the not operand, and possibly grouped into orblocks.

     The following match patterns can be used (listed in alphabetical order):

     // this is a comment.
	     Inserts the specified text as a comment in the rule.  Everything
	     following // is considered as a comment and stored in the rule.
	     You can have comment‐only rules, which are listed as having a
	     the IP stack output for delivery.

     dstip ipaddress
	     Matches IPv4 packets whose destination IP is one of the
	     address(es) specified as argument.

     {dstip6 | dstipv6} ip6address
	     Matches IPv6 packets whose destination IP is one of the
	     address(es) specified as argument.

     dstport ports
	     Matches IP packets whose destination port is one of the port(s)
	     specified as argument.

	     Matches TCP packets that have the RST or ACK bits set.

     ext6hdr header
	     Matches IPv6 packets containing the extended header given by
	     header.  Supported headers are:

	     Fragment, (frag), Hop‐to‐hop options (hopopt), any type of Rout‐
	     ing Header (route), Source routing Routing Header Type 0
	     (rthdr0), Mobile IPv6 Routing Header Type 2 (rthdr2), Destination
	     options (dstopt), IPSec authentication headers (ah), and IPsec
	     encapsulated security payload headers (esp).

     fib fibnum
	     Matches a packet that has been tagged to use the given FIB (rout‐
	     ing table) number.

     flowid labels
	     Matches IPv6 packets containing any of the flow labels given in
	     labels.  labels is a comma separated list of numeric flow labels.

     frag    Matches packets that are fragments and not the first fragment of
	     an IP datagram.  Note that these packets will not have the next
	     protocol header (e.g. TCP, UDP) so options that look into these
	     headers cannot match.

     gid group
	     Matches all TCP or UDP packets sent by or received for a group.
	     A group may be specified by name or number.

     jail prisonID
	     Matches all TCP or UDP packets sent by or received for the jail
	     whos prison ID is prisonID.

     icmptypes types
	     Matches ICMP packets whose ICMP type is in the list types.  The
	     list may be specified as any combination of individual types
	     (numeric) separated by commas.  Ranges are not allowed.  The sup‐

     in | out
	     Matches incoming or outgoing packets, respectively.  in and out
	     are mutually exclusive (in fact, out is implemented as not in).

     ipid idlist
	     Matches IPv4 packets whose ip_id field has value included in
	     idlist, which is either a single value or a list of values or
	     ranges specified in the same way as ports.

     iplen lenlist
	     Matches IP packets whose total length, including header and data,
	     is in the set lenlist, which is either a single value or a list
	     of values or ranges specified in the same way as ports.

     ipoptions spec
	     Matches packets whose IPv4 header contains the comma separated
	     list of options specified in spec.  The supported IP options are:

	     ssrr (strict source route), lsrr (loose source route), rr (record
	     packet route) and ts (timestamp).	The absence of a particular
	     option may be denoted with a ‘!’.

     ipprecedence precedence
	     Matches IPv4 packets whose precedence field is equal to

     ipsec   Matches packets that have IPSEC history associated with them
	     (i.e., the packet comes encapsulated in IPSEC, the kernel has
	     IPSEC support and IPSEC_FILTERTUNNEL option, and can correctly
	     decapsulate it).

	     Note that specifying ipsec is different from specifying proto
	     ipsec as the latter will only look at the specific IP protocol
	     field, irrespective of IPSEC kernel support and the validity of
	     the IPSEC data.

	     Further note that this flag is silently ignored in kernels with‐
	     out IPSEC support.  It does not affect rule processing when given
	     and the rules are handled as if with no ipsec flag.

     iptos spec
	     Matches IPv4 packets whose tos field contains the comma separated
	     list of service types specified in spec.  The supported IP types
	     of service are:

	     lowdelay (IPTOS_LOWDELAY), throughput (IPTOS_THROUGHPUT),
	     reliability (IPTOS_RELIABILITY), mincost (IPTOS_MINCOST),
	     congestion (IPTOS_ECN_CE).  The absence of a particular type may
	     be denoted with a ‘!’.

     ipttl ttllist
	     Matches IPv4 packets whose time to live is included in ttllist,

     layer2  Matches only layer2 packets, i.e., those passed to ipfw from
	     ether_demux() and ether_output_frame().

     limit {srcaddr | srcport | dstaddr | dstport} N
	     The firewall will only allow N connections with the same set of
	     parameters as specified in the rule.  One or more of source and
	     destination addresses and ports can be specified.	Currently,
	     only IPv4 flows are supported.

     { MAC | mac } dstmac srcmac
	     Match packets with a given dstmac and srcmac addresses, speci‐
	     fied as the any keyword (matching any MAC address), or six groups
	     of hex digits separated by colons, and optionally followed by a
	     mask indicating the significant bits.  The mask may be specified
	     using either of the following methods:

	     1.      A slash (/) followed by the number of significant bits.
		     For example, an address with 33 significant bits could be
		     specified as:

			   MAC 10:20:30:40:50:60/33 any

	     2.      An ampersand (&) followed by a bitmask specified as six
		     groups of hex digits separated by colons.	For example,
		     an address in which the last 16 bits are significant
		     could be specified as:

			   MAC 10:20:30:40:50:60&00:00:00:00:ff:ff any

		     Note that the ampersand character has a special meaning
		     in many shells and should generally be escaped.

	     Note that the order of MAC addresses (destination first, source
	     second) is the same as on the wire, but the opposite of the one
	     used for IP addresses.

     mactype mactype
	     Matches packets whose Ethernet Type field corresponds to one of
	     those specified as argument.  mactype is specified in the same
	     way as port numbers (i.e., one or more comma‐separated single
	     values or ranges).  You can use symbolic names for known values
	     such as vlan, ipv4, ipv6.	Values can be entered as decimal or
	     hexadecimal (if prefixed by 0x), and they are always printed as
	     hexadecimal (unless the N option is used, in which case symbolic
	     resolution will be attempted).

     proto protocol
	     Matches packets with the corresponding IP protocol.

     recv | xmit | via {ifX | if* | ipno | any}
	     Matches packets received, transmitted or going through, respec‐
	     tively, the interface specified by exact name (ifX), by device

	     A packet may not have a receive or transmit interface: packets
	     originating from the local host have no receive interface, while
	     packets destined for the local host have no transmit interface.

     setup   Matches TCP packets that have the SYN bit set but no ACK bit.
	     This is the short form of “tcpflags syn,!ack”.

     srcip ipaddress
	     Matches IPv4 packets whose source IP is one of the address(es)
	     specified as an argument.

     srcip6 ip6address
	     Matches IPv6 packets whose source IP is one of the address(es)
	     specified as an argument.

     srcport ports
	     Matches IP packets whose source port is one of the port(s) speci‐
	     fied as argument.

     tagged taglist
	     Matches packets whose tags are included in taglist, which is
	     either a single value or a list of values or ranges specified in
	     the same way as ports.  Tags can be applied to the packet using
	     tag rule action parameter (see it’s description for details on

     tcpack ack
	     TCP packets only.	Match if the TCP header acknowledgment number
	     field is set to ack.

     tcpdatalen tcpdatalenlist
	     Matches TCP packets whose length of TCP data is tcpdatalenlist,
	     which is either a single value or a list of values or ranges
	     specified in the same way as ports.

     tcpflags spec
	     TCP packets only.	Match if the TCP header contains the comma
	     separated list of flags specified in spec.  The supported TCP
	     flags are:

	     fin, syn, rst, psh, ack and urg.  The absence of a particular
	     flag may be denoted with a ‘!’.  A rule which contains a tcpflags
	     specification can never match a fragmented packet which has a
	     non‐zero offset.  See the frag option for details on matching
	     fragmented packets.

     tcpseq seq
	     TCP packets only.	Match if the TCP header sequence number field
	     is set to seq.

     tcpwin win
	     Match all TCP or UDP packets sent by or received for a user.  A
	     user may be matched by name or identification number.

	     For incoming packets, a routing table lookup is done on the
	     packet’s source address.  If the interface on which the packet
	     entered the system matches the outgoing interface for the route,
	     the packet matches.  If the interfaces do not match up, the
	     packet does not match.  All outgoing packets or packets with no
	     incoming interface match.

	     The name and functionality of the option is intentionally similar
	     to the Cisco IOS command:

		   ip verify unicast reverse‐path

	     This option can be used to make anti‐spoofing rules to reject all
	     packets with source addresses not from this interface.  See also
	     the option antispoof.

	     For incoming packets, a routing table lookup is done on the
	     packet’s source address.  If a route to the source address
	     exists, but not the default route or a blackhole/reject route,
	     the packet matches.  Otherwise, the packet does not match.  All
	     outgoing packets match.

	     The name and functionality of the option is intentionally similar
	     to the Cisco IOS command:

		   ip verify unicast source reachable‐via any

	     This option can be used to make anti‐spoofing rules to reject all
	     packets whose source address is unreachable.

	     For incoming packets, the packet’s source address is checked if
	     it belongs to a directly connected network.  If the network is
	     directly connected, then the interface the packet came on in is
	     compared to the interface the network is connected to.  When
	     incoming interface and directly connected interface are not the
	     same, the packet does not match.  Otherwise, the packet does
	     match.  All outgoing packets match.

	     This option can be used to make anti‐spoofing rules to reject all
	     packets that pretend to be from a directly connected network but
	     do not come in through that interface.  This option is similar to
	     but more restricted than verrevpath because it engages only on
	     packets with source addresses of directly connected networks
	     instead of all source addresses.


     Internally, each table is stored in a Radix tree, the same way as the
     routing table (see route(4)).

     Lookup tables currently support IPv4 addresses only.

     The tablearg feature provides the ability to use a value, looked up in
     the table, as the argument for a rule action, action parameter or rule
     option.  This can significantly reduce number of rules in some configura‐
     tions.  If two tables are used in a rule, the result of the second (des‐
     tination) is used.  The tablearg argument can be used with the following
     actions: nat, pipe, queue, divert, tee, netgraph, ngtee, fwd, skipto
     action parameters: tag, untag, rule options: limit, tagged.

     When used with fwd it is possible to supply table entries with values
     that are in the form of IP addresses or hostnames.  See the EXAMPLES Sec‐
     tion for example usage of tables and the tablearg keyword.

     When used with the skipto action, the user should be aware that the code
     will walk the ruleset up to a rule equal to, or past, the given number,
     and should therefore try keep the ruleset compact between the skipto and
     the target rules.


     Each rule belongs to one of 32 different sets , numbered 0 to 31.	Set 31
     is reserved for the default rule.

     By default, rules are put in set 0, unless you use the set N attribute
     when entering a new rule.	Sets can be individually and atomically
     enabled or disabled, so this mechanism permits an easy way to store mul‐
     tiple configurations of the firewall and quickly (and atomically) switch
     between them.  The command to enable/disable sets is

	   ipfw set [disable number ...] [enable number ...]

     where multiple enable or disable sections can be specified.  Command exe‐
     cution is atomic on all the sets specified in the command.  By default,
     all sets are enabled.

     When you disable a set, its rules behave as if they do not exist in the
     firewall configuration, with only one exception:

	   dynamic rules created from a rule before it had been disabled will
	   still be active until they expire.  In order to delete dynamic
	   rules you have to explicitly delete the parent rule which generated

     The set number of rules can be changed with the command

	   ipfw set move {rule rulenumber | oldset} to newset

     Also, you can atomically swap two rulesets with the command
     dstip/dstport pair of addresses (src and dst are used here only to
     denote the initial match addresses, but they are completely equivalent
     afterwards).  Dynamic rules will be checked at the first checkstate,
     keepstate or limit occurrence, and the action performed upon a match
     will be the same as in the parent rule.

     Note that no additional attributes other than protocol and IP addresses
     and ports are checked on dynamic rules.

     The typical use of dynamic rules is to keep a closed firewall configura‐
     tion, but let the first TCP SYN packet from the inside network install a
     dynamic rule for the flow so that packets belonging to that session will
     be allowed through the firewall:

	   ipfw add check‐state
	   ipfw add allow tcp from my‐subnet to any setup keep‐state
	   ipfw add deny tcp from any to any

     A similar approach can be used for UDP, where an UDP packet coming from
     the inside will install a dynamic rule to let the response through the

	   ipfw add check‐state
	   ipfw add allow udp from my‐subnet to any keep‐state
	   ipfw add deny udp from any to any

     Dynamic rules expire after some time, which depends on the status of the
     flow and the setting of some sysctl variables.  See Section SYSCTL
     VARIABLES for more details.  For TCP sessions, dynamic rules can be
     instructed to periodically send keepalive packets to refresh the state of
     the rule when it is about to expire.

     See Section EXAMPLES for more examples on how to use dynamic rules.


     ipfw is also the user interface for the dummynet traffic shaper and net‐
     work emulator, a subsystem that can artificially queue, delay or drop
     packets emulator the behaviour of certain network links or queueing sys‐

     dummynet operates by first using the firewall to select packets using any
     match pattern that can be used in ipfw rules.  Matching packets are then
     passed to either of two different objects, which implement the traffic

	 pipe	 A pipe emulates a link with given bandwidth, propagation
		 delay, queue size and packet loss rate.  Packets are queued
		 in front of the pipe as they come out from the classifier,
		 and then transferred to the pipe according to the pipe’s

	 queue	 A queue is an abstraction used to implement the WF2Q+ (Worst‐
     share the available bandwidth.

     There are two modes of dummynet operation: “normal” and “fast”.  The
     “normal” mode tries to emulate a real link: the dummynet scheduler
     ensures that the packet will not leave the pipe faster than it would on
     the real link with a given bandwidth.  The “fast” mode allows certain
     packets to bypass the dummynet scheduler (if packet flow does not exceed
     pipe’s bandwidth).  This is the reason why the “fast” mode requires less
     CPU cycles per packet (on average) and packet latency can be signifi‐
     cantly lower in comparison to a real link with the same bandwidth.  The
     default mode is “normal”.	The “fast” mode can be enabled by setting the
     net.inet.ip.dummynet.io_fast sysctl(8) variable to a non‐zero value.

     The pipe and queue configuration commands are the following:

	   pipe number config pipeconfiguration

	   queue number config queueconfiguration

     The following parameters can be configured for a pipe:

     bw bandwidth | device
	     Bandwidth, measured in [K|M]{bit/s|Byte/s}.

	     A value of 0 (default) means unlimited bandwidth.	The unit must
	     immediately follow the number, as in

		   ipfw pipe 1 config bw 300Kbit/s

	     If a device name is specified instead of a numeric value, as in

		   ipfw pipe 1 config bw tun0

	     then the transmit clock is supplied by the specified device.  At
	     the moment only the tun(4) device supports this functionality,
	     for use in conjunction with ppp(8).

     delay msdelay
	     Propagation delay, measured in milliseconds.  The value is
	     rounded to the next multiple of the clock tick (typically 10ms,
	     but it is a good practice to run kernels with “options HZ=1000”
	     to reduce the granularity to 1ms or less).  The default value is
	     0, meaning no delay.

     burst size
	     If the data to be sent exceeds the pipe’s bandwidth limit (and
	     the pipe was previously idle), up to size bytes of data are
	     allowed to bypass the dummynet scheduler, and will be sent as
	     fast as the physical link allows.	Any additional data will be
	     transmitted at the rate specified by the pipe bandwidth.  The
	     burst size depends on how long the pipe has been idle; the effec‐
	     the additional delay with an empirical curve that represents its

			 cumulative probability
			 1.0 ^
			 L   +‐‐ loss‐level	     x
			     |		       ******
			     |		      *
			     |		 *****
			     |		*
			     |	      **
			     |	     *
	     The empirical curve may have both vertical and horizontal lines.
	     Vertical lines represent constant delay for a range of probabili‐
	     ties.  Horizontal lines correspond to a discontinuity in the
	     delay distribution: the pipe will use the largest delay for a
	     given probability.

	     The file format is the following, with whitespace acting as a
	     separator and ’#’ indicating the beginning a comment:

	     name identifier
		     optional name (listed by "ipfw pipe show") to identify
		     the delay distribution;

	     bw value
		     the bandwidth used for the pipe.  If not specified here,
		     it must be present explicitly as a configuration parame‐
		     ter for the pipe;

	     losslevel L
		     the probability above which packets are lost.  (0.0 <= L
		     <= 1.0, default 1.0 i.e. no loss);

	     samples N
		     the number of samples used in the internal representation
		     of the curve (2..1024; default 100);

	     delay prob | prob delay
		     One of these two lines is mandatory and defines the for‐
		     mat of the following lines with data points.

	     XXX YYY
		     2 or more lines representing points in the curve, with
		     either delay or probability first, according to the cho‐
		     sen format.  The unit for delay is milliseconds.  Data
		     points do not need to be sorted.  Also, tne number of
		     actual lines can be different from the value of the "sam‐
		     ples" parameter: ipfw utility will sort and interpolate

		   #configuration file end

     The following parameters can be configured for a queue:

     pipe pipe_nr
	     Connects a queue to the specified pipe.  Multiple queues (with
	     the same or different weights) can be connected to the same pipe,
	     which specifies the aggregate rate for the set of queues.

     weight weight
	     Specifies the weight to be used for flows matching this queue.
	     The weight must be in the range 1..100, and defaults to 1.

     Finally, the following parameters can be configured for both pipes and

     buckets hashtablesize
	   Specifies the size of the hash table used for storing the various
	   queues.  Default value is 64 controlled by the sysctl(8) variable
	   net.inet.ip.dummynet.hash_size, allowed range is 16 to 65536.

     mask maskspecifier
	   Packets sent to a given pipe or queue by an ipfw rule can be fur‐
	   ther classified into multiple flows, each of which is then sent to
	   a different dynamic pipe or queue.  A flow identifier is con‐
	   structed by masking the IP addresses, ports and protocol types as
	   specified with the mask options in the configuration of the pipe or
	   queue.  For each different flow identifier, a new pipe or queue is
	   created with the same parameters as the original object, and match‐
	   ing packets are sent to it.

	   Thus, when dynamic pipes are used, each flow will get the same
	   bandwidth as defined by the pipe, whereas when dynamic queues are
	   used, each flow will share the parent’s pipe bandwidth evenly with
	   other flows generated by the same queue (note that other queues
	   with different weights might be connected to the same pipe).
	   Available mask specifiers are a combination of one or more of the

	   dstip mask, dstip6 mask, srcip mask, srcip6 mask, dstport
	   mask, srcport mask, flowid mask, proto mask or all,

	   where the latter means all bits in all fields are significant.

	   When a packet is dropped by a dummynet queue or pipe, the error is
	   normally reported to the caller routine in the kernel, in the same
	   way as it happens when a device queue fills up.  Setting this
	   option reports the packet as successfully delivered, which can be
	   needed for some experimental setups where you want to simulate loss
	   or congestion at a remote router.

	   net.inet.ip.dummynet.pipe_byte_limit and
	   net.inet.ip.dummynet.pipe_slot_limit control the maximum lengths
	   that can be specified.

     red | gred w_q/min_th/max_th/max_p
	   Make use of the RED (Random Early Detection) queue management algo‐
	   rithm.  w_q and max_p are floating point numbers between 0 and 1 (0
	   not included), while min_th and max_th are integer numbers specify‐
	   ing thresholds for queue management (thresholds are computed in
	   bytes if the queue has been defined in bytes, in slots otherwise).
	   The dummynet also supports the gentle RED variant (gred).  Three
	   sysctl(8) variables can be used to control the RED behaviour:

		   specifies the accuracy in computing the average queue when
		   the link is idle (defaults to 256, must be greater than

		   specifies the expected average packet size (defaults to
		   512, must be greater than zero)

		   specifies the expected maximum packet size, only used when
		   queue thresholds are in bytes (defaults to 1500, must be
		   greater than zero).

     When used with IPv6 data, dummynet currently has several limitations.
     Information necessary to route link‐local packets to an interface is not
     available after processing by dummynet so those packets are dropped in
     the output path.  Care should be taken to insure that link‐local packets
     are not passed to dummynet.


     Here are some important points to consider when designing your rules:

     	 Remember that you filter both packets going in and out.  Most connec‐
	 tions need packets going in both directions.

     	 Remember to test very carefully.  It is a good idea to be near the
	 console when doing this.  If you cannot be near the console, use an
	 auto‐recovery script such as the one in

     	 Do not forget the loopback interface.


     	 There are circumstances where fragmented datagrams are uncondition‐
	 ally dropped.	TCP packets are dropped if they do not contain at
	 least 20 bytes of TCP header, UDP packets are dropped if they do not
	 contain a full 8 byte UDP header, and ICMP packets are dropped if
	 they do not contain 4 bytes of ICMP header, enough to specify the
	       ipfw add 32000 allow ip from any to any

	 Along the same lines, doing an

	       ipfw flush

	 in similar surroundings is also a bad idea.

     	 The ipfw filter list may not be modified if the system security level
	 is set to 3 or higher (see init(8) for information on system security


     A divert(4) socket bound to the specified port will receive all packets
     diverted to that port.  If no socket is bound to the destination port, or
     if the divert module is not loaded, or if the kernel was not compiled
     with divert socket support, the packets are dropped.


     ipfw support in‐kernel NAT using the kernel version of libalias(3).

     The nat configuration command is the following:

	   nat nat_number config natconfiguration

     The following parameters can be configured:

     ip ip_address
	     Define an ip address to use for aliasing.

     if nic  Use ip address of NIC for aliasing, dynamically changing it if
	     NIC’s ip address changes.

     log     Enable logging on this nat instance.

	     Deny any incoming connection from outside world.

	     Try to leave the alias port numbers unchanged from the actual
	     local port numbers.

	     Traffic on the local network not originating from an unregistered
	     address spaces will be ignored.

     reset   Reset table of the packet aliasing engine on address change.

	     Reverse the way libalias handles aliasing.

     command line tool.  The main difference is that sctp nat does not do port
     translation.  Since the local and global side ports will be the same,
     there is no need to specify both.	Ports are redirected as follows:

	   nat nat_number config if nic redirect_port sctp
	   ip_address [,addr_list] {[port | portport] [,ports]}

     Most sctp nat configuration can be done in real‐time through the
     sysctl(8) interface.  All may be changed dynamically, though the hash_ta‐
     ble size will only change for new nat instances.  See SYSCTL VARIABLES
     for more info.


     A set of sysctl(8) variables controls the behaviour of the firewall and
     associated modules (dummynet, bridge, sctp nat).  These are shown below
     together with their default value (but always check with the sysctl(8)
     command what value is actually in use) and meaning:

     net.inet.ip.alias.sctp.accept_global_ootb_addip: 0
	     Defines how the nat responds to receipt of global OOTB ASCONF‐

	     0	     No response (unless a partially matching association
		     exists ‐ ports and vtags match but global address does

	     1	     nat will accept and process all OOTB global AddIP mes‐

	     Option 1 should never be selected as this forms a security risk.
	     An attacker can establish multiple fake associations by sending
	     AddIP messages.

     net.inet.ip.alias.sctp.chunk_proc_limit: 5
	     Defines the maximum number of chunks in an SCTP packet that will
	     be parsed for a packet that matches an existing association.
	     This value is enforced to be greater or equal than
	     net.inet.ip.alias.sctp.initialising_chunk_proc_limit.  A high
	     value is a DoS risk yet setting too low a value may result in
	     important control chunks in the packet not being located and

     net.inet.ip.alias.sctp.error_on_ootb: 1
	     Defines when the nat responds to any Out‐of‐the‐Blue (OOTB) pack‐
	     ets with ErrorM packets.  An OOTB packet is a packet that arrives
	     with no existing association registered in the nat and is not an
	     INIT or ASCONF‐AddIP packet:

	     0	     ErrorM is never sent in response to OOTB packets.

	     1	     ErrorM is only sent to OOTB packets received on the local

	     allow global hosts to be informed when they need to (re)send an
	     ASCONF‐AddIP.  Value 3 should never be chosen (except for debug‐
	     ging) as the nat will respond to all OOTB global packets (a DoS

     net.inet.ip.alias.sctp.hashtable_size: 2003
	     Size of hash tables used for nat lookups (100 < prime_number >
	     1000001).	This value sets the hash table size for any future
	     created nat instance and therefore must be set prior to creating
	     a nat instance.  The table sizes may be changed to suit specific
	     needs.  If there will be few concurrent associations, and memory
	     is scarce, you may make these smaller.  If there will be many
	     thousands (or millions) of concurrent associations, you should
	     make these larger.  A prime number is best for the table size.
	     The sysctl update function will adjust your input value to the
	     next highest prime number.

     net.inet.ip.alias.sctp.holddown_time: 0
	     Hold association in table for this many seconds after receiving a
	     SHUTDOWN‐COMPLETE.  This allows endpoints to correct shutdown
	     gracefully if a shutdown_complete is lost and retransmissions are

     net.inet.ip.alias.sctp.init_timer: 15
	     Timeout value while waiting for (INIT‐ACK|AddIP‐ACK).  This value
	     cannot be 0.

     net.inet.ip.alias.sctp.initialising_chunk_proc_limit: 2
	     Defines the maximum number of chunks in an SCTP packet that will
	     be parsed when no existing association exists that matches that
	     packet.  Ideally this packet will only be an INIT or ASCONF‐AddIP
	     packet.  A higher value may become a DoS risk as malformed pack‐
	     ets can consume processing resources.

     net.inet.ip.alias.sctp.param_proc_limit: 25
	     Defines the maximum number of parameters within a chunk that will
	     be parsed in a packet.  As for other similar sysctl variables,
	     larger values pose a DoS risk.

     net.inet.ip.alias.sctp.log_level: 0
	     Level of detail in the system log messages (0 - minimal, 1 -
	     event, 2 - info, 3 - detail, 4 - debug, 5 - max debug). May be a
	     good option in high loss environments.

     net.inet.ip.alias.sctp.shutdown_time: 15
	     Timeout value while waiting for SHUTDOWN‐COMPLETE.  This value
	     cannot be 0.

     net.inet.ip.alias.sctp.track_global_addresses: 0
	     Enables/disables global IP address tracking within the nat and
	     places an upper limit on the number of addresses tracked for each

     net.inet.ip.alias.sctp.up_timer: 300
	     Timeout value to keep an association up with no traffic.  This
	     value cannot be 0.

     net.inet.ip.dummynet.expire: 1
	     Lazily delete dynamic pipes/queue once they have no pending traf‐
	     fic.  You can disable this by setting the variable to 0, in which
	     case the pipes/queues will only be deleted when the threshold is

     net.inet.ip.dummynet.hash_size: 64
	     Default size of the hash table used for dynamic pipes/queues.
	     This value is used when no buckets option is specified when con‐
	     figuring a pipe/queue.

     net.inet.ip.dummynet.io_fast: 0
	     If set to a non‐zero value, the “fast” mode of dummynet operation
	     (see above) is enabled.

	     Number of packets passed to dummynet.

	     Number of packets dropped by dummynet.

	     Number of packets bypassed by the dummynet scheduler.

     net.inet.ip.dummynet.max_chain_len: 16
	     Target value for the maximum number of pipes/queues in a hash
	     bucket.  The product max_chain_len*hash_size is used to determine
	     the threshold over which empty pipes/queues will be expired even
	     when net.inet.ip.dummynet.expire=0.

     net.inet.ip.dummynet.red_lookup_depth: 256

     net.inet.ip.dummynet.red_avg_pkt_size: 512

     net.inet.ip.dummynet.red_max_pkt_size: 1500
	     Parameters used in the computations of the drop probability for
	     the RED algorithm.

     net.inet.ip.dummynet.pipe_byte_limit: 1048576

     net.inet.ip.dummynet.pipe_slot_limit: 100
	     The maximum queue size that can be specified in bytes or packets.
	     These limits prevent accidental exhaustion of resources such as
	     mbufs.  If you raise these limits, you should make sure the sys‐
	     tem is configured so that sufficient resources are available.

     net.inet.ip.fw.autoinc_step: 100

     net.inet.ip.fw.dyn_buckets: 256
	     The number of buckets in the hash table for dynamic rules.  Must
	     be a power of 2, up to 65536.  It only takes effect when all
	     dynamic rules have expired, so you are advised to use a flush
	     command to make sure that the hash table is resized.

     net.inet.ip.fw.dyn_count: 3
	     Current number of dynamic rules (read‐only).

     net.inet.ip.fw.dyn_keepalive: 1
	     Enables generation of keepalive packets for keepstate rules on
	     TCP sessions.  A keepalive is generated to both sides of the con‐
	     nection every 5 seconds for the last 20 seconds of the lifetime
	     of the rule.

     net.inet.ip.fw.dyn_max: 8192
	     Maximum number of dynamic rules.  When you hit this limit, no
	     more dynamic rules can be installed until old ones expire.

     net.inet.ip.fw.dyn_ack_lifetime: 300

     net.inet.ip.fw.dyn_syn_lifetime: 20

     net.inet.ip.fw.dyn_fin_lifetime: 1

     net.inet.ip.fw.dyn_rst_lifetime: 1

     net.inet.ip.fw.dyn_udp_lifetime: 5

     net.inet.ip.fw.dyn_short_lifetime: 30
	     These variables control the lifetime, in seconds, of dynamic
	     rules.  Upon the initial SYN exchange the lifetime is kept short,
	     then increased after both SYN have been seen, then decreased
	     again during the final FIN exchange or when a RST is received.
	     Both dyn_fin_lifetime and dyn_rst_lifetime must be strictly lower
	     than 5 seconds, the period of repetition of keepalives.  The
	     firewall enforces that.

     net.inet.ip.fw.enable: 1
	     Enables the firewall.  Setting this variable to 0 lets you run
	     your machine without firewall even if compiled in.

     net.inet6.ip6.fw.enable: 1
	     provides the same functionality as above for the IPv6 case.

     net.inet.ip.fw.one_pass: 1
	     When set, the packet exiting from the dummynet pipe or from
	     ng_ipfw(4) node is not passed though the firewall again.  Other‐
	     wise, after an action, the packet is reinjected into the firewall
	     at the next rule.

	     Controls whether layer‐2 packets are passed to ipfw.  Default is

     net.link.bridge.ipfw: 0
	     Controls whether bridged packets are passed to ipfw.  Default is


     There are far too many possible uses of ipfw so this Section will only
     give a small set of examples.

     This command adds an entry which denies all tcp packets from
     cracker.evil.org to the telnet port of wolf.tambov.su from being for‐
     warded by the host:

	   ipfw add deny tcp from cracker.evil.org to wolf.tambov.su telnet

     This one disallows any connection from the entire cracker’s network to my

	   ipfw add deny ip from to my.host.org

     A first and efficient way to limit access (not using dynamic rules) is
     the use of the following rules:

	   ipfw add allow tcp from any to any established
	   ipfw add allow tcp from net1 portlist1 to net2 portlist2 setup
	   ipfw add allow tcp from net3 portlist3 to net3 portlist3 setup
	   ipfw add deny tcp from any to any

     The first rule will be a quick match for normal TCP packets, but it will
     not match the initial SYN packet, which will be matched by the setup
     rules only for selected source/destination pairs.	All other SYN packets
     will be rejected by the final deny rule.

     If you administer one or more subnets, you can take advantage of the
     address sets and or‐blocks and write extremely compact rulesets which
     selectively enable services to blocks of clients, as below:

	   goodguys="{{20,35,66,18} or{6,3,11} }"

	   ipfw add allow ip from ${goodguys} to any
	   ipfw add deny ip from ${badguys} to any
	   ... normal policies ...

     The verrevpath option could be used to do automated anti‐spoofing by
     adding the following to the top of a ruleset:

	   ipfw add deny ip from any to any not verrevpath in
     fxp0, but coming in on fxp1 would be dropped.

     In order to protect a site from flood attacks involving fake TCP packets,
     it is safer to use dynamic rules:

	   ipfw add check‐state
	   ipfw add deny tcp from any to any established
	   ipfw add allow tcp from my‐net to any setup keep‐state

     This will let the firewall install dynamic rules only for those connec‐
     tion which start with a regular SYN packet coming from the inside of our
     network.  Dynamic rules are checked when encountering the first
     checkstate or keepstate rule.  A checkstate rule should usually be
     placed near the beginning of the ruleset to minimize the amount of work
     scanning the ruleset.  Your mileage may vary.

     To limit the number of connections a user can open you can use the fol‐
     lowing type of rules:

	   ipfw add allow tcp from my‐net/24 to any setup limit src‐addr 10
	   ipfw add allow tcp from any to me setup limit src‐addr 4

     The former (assuming it runs on a gateway) will allow each host on a /24
     network to open at most 10 TCP connections.  The latter can be placed on
     a server to make sure that a single client does not use more than 4
     simultaneous connections.

     BEWARE: stateful rules can be subject to denial‐of‐service attacks by a
     SYN‐flood which opens a huge number of dynamic rules.  The effects of
     such attacks can be partially limited by acting on a set of sysctl(8)
     variables which control the operation of the firewall.

     Here is a good usage of the list command to see accounting records and
     timestamp information:

	   ipfw ‐at list

     or in short form without timestamps:

	   ipfw ‐a list

     which is equivalent to:

	   ipfw show

     Next rule diverts all incoming packets from to divert port

	   ipfw divert 5000 ip from to any in


	   ipfw add pipe 1 ip from to any out
	   ipfw pipe 1 config bw 300Kbit/s queue 50KBytes

     note that we use the out modifier so that the rule is not used twice.
     Remember in fact that ipfw rules are checked both on incoming and outgo‐
     ing packets.

     Should we want to simulate a bidirectional link with bandwidth limita‐
     tions, the correct way is the following:

	   ipfw add pipe 1 ip from any to any out
	   ipfw add pipe 2 ip from any to any in
	   ipfw pipe 1 config bw 64Kbit/s queue 10Kbytes
	   ipfw pipe 2 config bw 64Kbit/s queue 10Kbytes

     The above can be very useful, e.g. if you want to see how your fancy Web
     page will look for a residential user who is connected only through a
     slow link.  You should not use only one pipe for both directions, unless
     you want to simulate a half‐duplex medium (e.g. AppleTalk, Ethernet,
     IRDA).  It is not necessary that both pipes have the same configuration,
     so we can also simulate asymmetric links.

     Should we want to verify network performance with the RED queue manage‐
     ment algorithm:

	   ipfw add pipe 1 ip from any to any
	   ipfw pipe 1 config bw 500Kbit/s queue 100 red 0.002/30/80/0.1

     Another typical application of the traffic shaper is to introduce some
     delay in the communication.  This can significantly affect applications
     which do a lot of Remote Procedure Calls, and where the round‐trip‐time
     of the connection often becomes a limiting factor much more than band‐

	   ipfw add pipe 1 ip from any to any out
	   ipfw add pipe 2 ip from any to any in
	   ipfw pipe 1 config delay 250ms bw 1Mbit/s
	   ipfw pipe 2 config delay 250ms bw 1Mbit/s

     Per‐flow queueing can be useful for a variety of purposes.  A very simple
     one is counting traffic:

	   ipfw add pipe 1 tcp from any to any
	   ipfw add pipe 1 udp from any to any
	   ipfw add pipe 1 ip from any to any
	   ipfw pipe 1 config mask all

     The above set of rules will create queues (and collect statistics) for
     all traffic.  Because the pipes have no limitations, the only effect is
     collecting statistics.  Note that we need 3 rules, not just the last one,
     In the following example, we need to create several traffic bandwidth
     classes and we need different hosts/networks to fall into different
     classes.  We create one pipe for each class and configure them accord‐
     ingly.  Then we create a single table and fill it with IP subnets and
     addresses.  For each subnet/host we set the argument equal to the number
     of the pipe that it should use.  Then we classify traffic using a single

	   ipfw pipe 1 config bw 1000Kbyte/s
	   ipfw pipe 4 config bw 4000Kbyte/s
	   ipfw table 1 add 1
	   ipfw table 1 add 4
	   ipfw table 1 add 1
	   ipfw add pipe tablearg ip from table(1) to any

     Using the fwd action, the table entries may include hostnames and IP

	   ipfw table 1 add
	   ipfw table 1 add router1.dmz
	   ipfw add 100 fwd tablearg ip from any to table(1)

     To add a set of rules atomically, e.g. set 18:

	   ipfw set disable 18
	   ipfw add NN set 18 ...	  # repeat as needed
	   ipfw set enable 18

     To delete a set of rules atomically the command is simply:

	   ipfw delete set 18

     To test a ruleset and disable it and regain control if something goes

	   ipfw set disable 18
	   ipfw add NN set 18 ...	  # repeat as needed
	   ipfw set enable 18; echo done; sleep 30 && ipfw set disable 18

     Here if everything goes well, you press control‐C before the "sleep" ter‐
     minates, and your ruleset will be left active.  Otherwise, e.g. if you
     cannot access your box, the ruleset will be disabled after the sleep ter‐
     minates thus restoring the previous situation.

     To show rules of the specific set:

	   ipfw set 18 show

	   ipfw add nat 123 all from any to any

     Then to configure nat instance 123 to alias all the outgoing traffic with
     ip, blocking all incoming connections, trying to keep same
     ports on both sides, clearing aliasing table on address change and keep‐
     ing a log of traffic/link statistics:

	   ipfw nat 123 config ip log deny_in reset same_ports

     Or to change address of instance 123, aliasing table will be cleared (see
     reset option):

	   ipfw nat 123 config ip

     To see configuration of nat instance 123:

	   ipfw nat 123 show config

     To show logs of all the instances in range 111‐999:

	   ipfw nat 111‐999 show

     To see configurations of all instances:

	   ipfw nat show config

     Or a redirect rule with mixed modes could looks like:

	   ipfw nat 123 config redirect_addr
			   redirect_port tcp 500
			   redirect_proto udp
			   redirect_addr, # LSNAT
			   redirect_port tcp,
				   500	      # LSNAT

     or it could be split in:

	   ipfw nat 1 config redirect_addr
	   ipfw nat 2 config redirect_port tcp 500
	   ipfw nat 3 config redirect_proto udp
	   ipfw nat 4 config redirect_addr,,
	   ipfw nat 5 config redirect_port tcp,, 500


     cpp(1), m4(1), altq(4), divert(4), dummynet(4), if_bridge(4), ip(4),
     ipfirewall(4), ng_ipfw(4), protocols(5), services(5), init(8),
     kldload(8), reboot(8), sysctl(8), syslogd(8)

     Dummynet has been introduced by Luigi Rizzo in 1997‐1998.

     Some early work (1999‐2000) on the dummynet traffic shaper supported by
     Akamba Corp.

     The ipfw core (ipfw2) has been completely redesigned and reimplemented by
     Luigi Rizzo in summer 2002. Further actions and options have been added
     by various developer over the years.

     In‐kernel NAT support written by Paolo Pisati ⟨piso@FreeBSD.org⟩ as part
     of a Summer of Code 2005 project.

     SCTP nat support has been developed by The Centre for Advanced Internet
     Architectures (CAIA) ⟨http://www.caia.swin.edu.au⟩.  The primary develop‐
     ers and maintainers are David Hayes and Jason But.  For further informa‐
     tion visit: ⟨http://www.caia.swin.edu.au/urp/SONATA⟩

     Delay profiles have been developed by Alessandro Cerri and Luigi Rizzo,
     supported by the European Commission within Projects Onelab and Onelab2.


     The syntax has grown over the years and sometimes it might be confusing.
     Unfortunately, backward compatibility prevents cleaning up mistakes made
     in the definition of the syntax.

     !!! WARNING !!!

     Misconfiguring the firewall can put your computer in an unusable state,
     possibly shutting down network services and requiring console access to
     regain control of it.

     Incoming packet fragments diverted by divert are reassembled before
     delivery to the socket.  The action used on those packet is the one from
     the rule which matches the first fragment of the packet.

     Packets diverted to userland, and then reinserted by a userland process
     may lose various packet attributes.  The packet source interface name
     will be preserved if it is shorter than 8 bytes and the userland process
     saves and reuses the sockaddr_in (as does natd(8)); otherwise, it may be
     lost.  If a packet is reinserted in this manner, later rules may be
     incorrectly applied, making the order of divert rules in the rule
     sequence very important.

     Dummynet drops all packets with IPv6 link‐local addresses.

     Rules using uid or gid may not behave as expected.  In particular, incom‐
     ing SYN packets may have no uid or gid associated with them since they do
     not yet belong to a TCP connection, and the uid/gid associated with a
     packet may not be as expected if the associated process calls setuid(2)
     or similar system calls.

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